Chromate treated metal sheet

ABSTRACT

ON A ELECTROLYTIC CHROMATE TREATED METAL SHEET HAVING A HYDRATE CHROMIC OXIDE LAYER WHICH IS ADJUSTED 0.1 TO 0.8 MG. CR./DM.2, 0.05 TO 2.5 MG. CR/DM.2 A FILM IS FORMED WHICH IS DUE TO THE WATER SOLUBLE RESIN. THIS RESIN IS AN ORGANIC FILM FORMING AGENT WHICH DOES NOT REDUCE A HEXAVAELNT CHROMIUM SUCH AS TRIASINE-FORMALINE RESIN, TITANIUM MODIFIED ACRYLIC RESIN OR CARBOXYLIC ACID MODIFIED ACRYLIC RESIN THEREBY PROVIDING A CHROMATE TREATED METAL SHEET OF EXCELLENT CORROSION RESISTANCE AND GOOD ADHESIVITY OF PAINT VARNISH, LACQUER AND OTHER ORGANIC FINISHES.

United States Patent O 3,799,814 CHROMATE TREATED METAL SHEET Hidehisa Yamagishi, Yokohama, Hiroshi Takano, Kawasaki, and Iwakichi Kawaguchi, Hiratsuka, Japan, asgignors to Nippon Kokan Kabushiki Kaisha, Tokyo,

apan No Drawing. Filed July 15, 1971, Ser. No. 165,777

Int. Cl. C23b 11/00 US. Cl. 1486.2 Claims ABSTRACT OF THE DISCLOSURE On an electrolytic chromate treated metal sheet having a hydrate chromic oxide layer which is adjusted 0.1 to 0.8 mg. Cr./dm. 0.05 to 2.5 mg. Cr/dm. a film is formed which is due to the water soluble resin. This resin is an organic film forming agent which does not reduce a hexavalent chromium such as triasine-formaline resin, titanium modified acrylic resin or carboxylic acid modified acrylic resin thereby providing a chromate treated metal sheet of excellent corrosion resistance and good adhesivity of paint varnish, lacquer and other organic finishes BACKGROUND OF THE INVENTION It has been known to improve on the corrosion resistance and appearance by forming metallic chromium film or duplex layer film of metallic chromium-hydrated chromium oxide on the surfaces of thin steel sheets or other metals, especially of steel sheet. The under mentioned may be taken for example of prior arts, that is,

(1) a process of forming very thin chromium film by carrying out the cathodic reduction in the surgent bath known as a standard chromium plating bath in which the sulphuric acid is added to about 1/ 100 of the chromic anhydride for 200 to 500 g./l. of the chromic anhydride.

(2) a process of further immersing the said surfacetreated sheet in the solution containing hexavalent chromium in the main or taking place the cathodic reduction in said solution, thereby forming chromium oxide being main of trivalent chromium on the first film.

(3) a process wherein the main agent is the chromate or the chromic acid of comparatively low concentration to 100 g./l.) to which various kinds of additives are used as assistants, and a duplex film consisting of metallic chromium-hydrated chromium oxide is formed through one treatment of the cathodic reduction with the assistants such as sulfate, silicofiuoride, borofiuoride or other being tartaric acid, maleic acid, oxalic acid, aromatic sulfonic acid having hydroxyl group, thiocyanate.

These conventional processes have obtained effective results. However the defect found commonly on the prior surface treating steel sheet is that although the hydrate chromic oxide of the trivalent chromium in the main existing in the 1st layer has the great effect to heighten the corrosion resistance, the thick layer thereof prevents the paint adhesion. On the contrary, when the chromium in the film on the sheet is almost to the metallic state, the corrosion resistance is inferior though the paint adhesion is excellent. Thus it is diificult to obtain corrosion resistance and the paint adhesion in conventional process. There have been proposed several processes to improve ice the surface properties of the metal sheet treated with such an electrolytic chromate, for example, French Pat. No. 1,452,202 where the hydrated chromium oxide is immersed in the solution of polycondensated products or the like of trimethyloctadecyl chloride ammonium formaldehyde and dicyanodiamide, however the processes are overlapping and complicated. Satisfactory results have not yet been obtained.

OUTLINE OF THE INVENTION The invention has succeeded in exactly solving the foregoing problems.

Throughout this specification the thickness of the chromate coating is generally given in term of mg. Cr/dmF, that is, milligrams of chromium (in the coating) per square decimeter.

According to the results of the inventors detailed investigations, in order that the said duplex electrolytic chromate treated steel sheet will have good corrosion resistance, it is necessary that the layer of the hydrated chromium oxide be more than 0.5 mg. Cr/dm. and the metallic chromium be more than 0.8 mg./dm. However the paint adhesion of this composition is not enough to endure the severe processing treatment such as forming a square can or screw-cap of said metal sheet. The amount of the hydrated chromium oxide, the better is the paint adhesion. When the amount of this hydrated chromium oxide is less than 0.2 mg. Cr/dm. it is possible to provide the strength of the paint adhesion which is enough to endure said processing treatment but the corrosion resistance is considerably weakened.

In regard to these circumstances, the inventors propose an improvement of the corrosion resistance by further forming an organic film on the duplex film of the thin hydrated chromium oxide layer which is excellent in the paint adhesion.

The basic object of the invention exists in forming a coating layer excellent in paint adhesion strength enough to endure the severe forming conditions such as forming corners of square cans or other vessels, the screw structure of the screw-cap, and in keeping suflicient corrosion resistance. As mentioned above, when the amount of the hydrated chromium oxide in the chromate film is decreased, good paint adhesion may be obtained, but on the other hand the corrosion resistance is very much lowered. The present invention forms the 3rd organic film on the thin hydrated chromium oxide layer to improve the corrosion resistance, thus obtaining the chromate treated sheet having satisfactory corrosion resistance and the paint adhesion to permit long term storage which is very significant industrially.

Another object of the invention is to economically obtain the chromate treated sheet of high commercial value. For this purpose, the hydrated chromium oxide is controlled to a suitable thickness, and the metallic chromium layer is controlled as well as the forming agent so that the organic film on the said hydrated chromium oxide layer is limited and furthermore its adhesion amount is controlled. In this manner, the colors are not taken on the treated sheet due to this organic film, and therefore the further paint coating layer on the metal sheet is clearly displayed to obtain cans of high art value. Moreover the pot life of the organic film forming agent is lengthened and the amount used is decreased to obtain economically the end product.

Other features or the actual effects will be apparent from the explanation of the embodiments of the invention.

DETAILED EXPLANATION The invention may use, as the metal sheet, various kinds such as zinc sheet, galvanized sheet, aluminium sheet or other, especially preferable is steel sheet, the thickness of which has not generally a limitation but is preferably 0.1 to mm., above all 0.2 to 0.5 mm. To these metal sheets the chromate treatment is undertaken With the prior optional process. The suitable manner thereof is to use as the main agent chromic acid or the chromate containing the hexavalent chromium concentration of to 100 g./l. of chromic anhydride, .to which one or more of such first assistants are added l/500 to 1/25 of said main agent as thiocyanic acid, thiosulfuric acid, dithionous acid, hydrogen sulfurous acid, sulfurous acid, thiosulfurous acid, pyrosulfurous acid or their salts or general sulfide, as well as one or more of such second assistant is used as a solution of inorganic fiuorin compound such as hydrofluoric acid silicofluoric acid borofluoric acid or any salts thereof of U500 to l/ 25 of said main agent (but this second assistant is not used when obtaining the comparatively thick hydrated chromium oxide by the chromate treatment, e.g. more than 0.5 mg. Cr/dm. The steel sheet or other metallic sheet is immersed in this solution to take place the cathodic electrolytic treatment and the hydrated chromium oxide layer is adjusted to provide the objective thickness for adhesion. The bath temperature for this chromate treatment is in general 50 to 60 C., the cathodic current density is 20 to 40 a./dm. and the electrolytic treating time is around 2 to 4 seconds. The thickness of the metallic chromium layer is in the invention 0.5 to 1.5 mg./dm. preferably 1.0 to 1.2 mg./dm. and in the case of less than 0.5 mg./dm. the corrosion resistance is inferior. Up to 1.5 mg./d1n. the corrosion resistance is practically enough and in the case of being more than that it is not economical. The thickness is the hydrated chromium oxide to be formed on the metallic chromium layer is in general 0.1 to 0.8 mg./dm. in the chromium amount, perferable 0.15 to 0.3 mg./dm. and in the case of less than 0.1 mg./dm. the corrosion resistance is inferior and in the case of more than 0.8 mgjdm. the workability is worse. As to the coating amount of said organic film it is in general 0.05 to 2 mg./dm. preferably 0.05 to 0.5 mg./dm. and even in the case of less than 0.05 mg./dm. the treatment shows the results as such but with less satisfactory corrosion resistance than expected, and in the case of more than 2 mg./dm. it is not economical, resulting in changing the color yellow.

The organic film forming solution applied to the invention is the treated solution for the organic film of the sole water soluble resin or the organic treating solution with addition of one or more of chromate, dichromate, molybdate, phosphate, titanate or oxyacid containing transition metals. The coating process is undertaken by immersing or coating the electrolytic chromate treated sheet in such solution and controlling if necessary, the film thickness by means of squeeze-rolling and drying the film with the hot air for a short period (e.g. around several seconds) or baking it for a very short time at a temperature of less than 200 C. The organic film formed according to such a process is at less than 2.5 mg./dm. which are remarkably thin in comparison with the amount of 45 to 150 mg./dm. in the conventional coating, being as thin as the conversion film and therefore even the treating solution of the sole resin could be taken for the organic film forming agent. The composition of such an organic treating solution may be as said above the sole water soluble resin with which the finishing of good color tone can be provided, and especially for the purpose of corrosion resistance it is preferable to use the above mentioned additives, especially the organic treating solution with addition of a little chromic acid, thereby to provide an improvement of corrosion resistance and further economically obtaining the thinner film. Phosphate, molybdate or titanate are superior to the sole resin in the corrosion resistance, though they are more or less inferior to chromic acid. The improvement of the corrosion resistance due to such additives are not always clarified theoretically, but it may be assumed to be due to the strong three dimensional structured film which is formed between the water soluble resin and the said salts in the drying or baking treatments after the above mentioned immersion. In this connection, when using additives there is a case that the solution is oxidized then depending upon the additives and in such a case the kinds of the water soluble resins are in fact modified not only from the fact that hexavalent chromium in the treated solution is dissolved, but also from the kinds of the additives. Consequently, one or more of the kinds of water soluble resin to be applied as film forming resin are limited to triazine-formaline resin, titanium modified acrylic resin or carboxylic acid modified acrylic resin which do not reduce the hexavalent chromium, and it is necessary to control pH of the aqueous solution in order to stabilize the organic film forming agent. pH is suitably controlled 5 to 10' in general, for which e.g. NH OH is used.

The actual embodiments of the producing process will be given. The invention is not, of course, limited to these embodiments or rather may be practiced in many cases according to these lines.

Example 1 The cold rolling steel sheet of 0.23 mm. thickness for tin plate was passed through the steps for electrolytic degreasing in C. orthosodasilicate, water washing, further electrolytic pickling with 50 g./l. sulfuric acid and water washing and after that the steel sheet was subjected to the cathodic electrolysis for 3 to 4 seconds at 20 to 30 a./dm. current density with the anode of lead (or leadantimony alloy or lead-tin alloy) in the solution of 50 g./l. chromic anhydride and 0.5 g./l. sodium thiocyanate, and by water washing and drying it several kinds of the duplex chromate treated steel sheets were obtained with the 1.2 to 1.5 mg./dm. metallic chromium amount and the hydrated chromium oxide in the 0.05 to 1.2 mg. Cr./dm.

The thus obtained chromate treated steel sheet had various kinds of color tones, from bluish luster to metallic luster, and were subsequently immersed in a treating solution of the under mentioned composition and conditlons.

Copolymer of itaconic acid and acrylnitrile: 0.5% Chromic anhydride: 0.8%

Ammonium hydroxide: Suitable quantity pH: 5 to 6 Temperature: 50 to 60 C.

The immersed steel sheet was passed through the squeeze-rolling to control the coating amount and dried with the 80 C. hot air for 2.5 to 10 seconds, to obtain the product with a coating amount of around 0.1 nllg/dmF, the film of which was transparent and very t in.

The product (steel sheet) of the invention was coated with the phenol-epoxy coating, and subjected to the formability test of the square can and the screw-cap. Table I shows the comparison of such products and the sheet not having been effected with the organic film treatment.

TABLE L-TEST OF COATING-ADHESIVE PROPERTIES (Phenol epoxy-coating 45 to 60 mgJdmJ) Chromium Coating (I) Coating (II) amount in hydrate Test of square-can Test of screw-cap Test of square-can Test of screw cap Total chromc iormability tormebility iormability tormability aint on e max.- Kinds of steel sheets (mg/dmfl) Inside Outside Inside Outside Inside Outside Inside Outside 80 point) Organic film treated sheet according to the invention 0. 1 10 10 8 8 10 10 10 10 76 0.2 10 10 7 8 10 10 10 10 75 0.5 7 9 6 6 10 10 9 10 67 0.7 6 8 6 6 10 10 9 10 64 According to the electrolytic chromate treat ment only 0. 8 3 6 3 6 10 10 9 10 67 Nora:

(1) Coating (I): Phenol-epoxy coating for fish can. Coating (II): (4) The estimating point for a t'ormability test of square can was Phenol-epoxy coating for beer can. that 10 point with no exfoliation at the four corners and when (2) In and out sides of coatings.

(3) Electrolytic chromate treated steel sheets of comparative sheets were equivalent or superior to known products in corrosion resistance. Accordingly hydrated chromium oxide was as much as 0.8 mg. Cr/dmfi.

As is seen from this Table I, when 0.8 mg. Cr/dm. exceeds the hydrated chromium oxide layer which is in the metallic chromium layer formed on the surface of the electrolytic chromate treated steel sheet, the results are almost the same as the known sheet even after the organic treatment of the invention. According to the inventors investigating results based on the various data, the amount of hydrated chromium oxide in the product effected with the organic treatment is generally around 0.1 to 0.8 mg. Cr/dmF, preferably 0.15 to 0.3 mg. Cr/dmF. The electrolytic chromate treated sheet having the organic treatment according to the invention has even less than 0.2 mg. Cr/dm. and shows the same corrosion resistance as that of the known steel sheet having 0.8 mg. Cr/dm. as is shown in Table II.

The treated steel sheets according to Example 1 were subjected to the humidity cabinet test at the temperature of 50 C. and 100% relative humidity for 24 hours so as to inspect and compare the rust-appearing condition, the results of which are shown in Table II. It is quite apparent that the organic treated sheets of the invention have more superior corrosion resistance to the more chromate treated sheets.

TABLE II.HUMIDITY CABINET TEST (24 HR.)

Chromium amount in hydrated Rust chromium appearing oxide area (mgJdmJ) (percent) NOTE.The estimating point of the light and heavy rust was that point was the case of no rust-appearing and demerit points were given exfoliation appeared in order [tom the portions of small radius curvature 2 demerit point was given per one corner, and when the exfoliation was )4 to the point was 0.

(5) In the iormability test of a screw-cap, the exfoliating condition in the screw portion of the screw-cap was estimated by the 0 to 10 point system in accordance to the standard sample.

Example 2 The electrolytic chromate treated sheets with 0.2 mg. Cr/dm. in the hydrated chromium oxide and 1.2

mg./dm. metallic chromium were treated in the organic film treating solution of the under mentioned composition.

Copolymer of itaconic acid and acrylonitrile percent 5 Chromic anhydride ..do 0.8 pH (adjusted by ammonium hydroxide) 5 to 6 According to the treating conditions of Example 1 the products with the coating amount of 0.5 mg./dm. were obtained.

Example 3 The electrolytic chromate treated sheets of the same composition as in Example 2 were treated in the solution of the under mentioned composition under the same conditions as in Example 1 and the products of 0.2 mg./dm.'* were obtained.

Copolymer of itaconic acid and acrylonitrile percent 0.5 Chromic anhydride ..do 0.1 pH (adjusted by ammonium hydroxide) 5 to 6 Example 4 Copolymer of itaconic acid and acrylnitrile according to the'rust-appearing degree. percent 0.5 The pointing standards are as f ll w Chromic anhydride d0 0.1 Norust 10 pH (adjusted by ammonium hydroxide) 5 to 6 Invisible to naked eye, vislble with magnifier 8 Faintly visible to naked eye 6 Table III shows the comparison of properties of the Clearly visible to naked eye 4 organic film treated sheets according to the invention Remarkable rust 1; obtained by Examples 1 to 4.

TABLE IIL-PROPERTY COME- ARISON IN EXAMPLES 1 T0 4 Composition of organic Corrosion film treating solution Coating adhesivity l resistance 9 Copolymer Square can Screw-cap Rust of itaconio tormability iormability appearacid and Chromic test test iug Light acrylnitrile anhydride area and (w In- Out- In- Out- (perheavy Kinds of steel sheet percent) percent) side side side side cent) rusts Example 2 6 0. 8 3 6 6 7 7 4 Example i 5 0. 1 6 8 6 7 64 1 Example 1 0. 5 0. 8 10 10 7 8 26 4 Example 4 0. 5 0. 1 7 9 6 6 V 64 4 According to the electrolytic treatment only i 3 6 4 4 4 1 Phenol epoxy system coating (1): 45 to 60 mgJdmJ. 1 Wetting test: 24 hrs.

Hydrated chromium oxide: 0.5 mg. Or/dmJ. Metallic chromium amount: 1.2 mg./dm.'.

In this table, the hydrated chromium oxide is 0.2 mg. Cr/dm. and the metallic chromium amount is 1.2 mg./ dm.

Example 5 The electrolytic chromate treated steel sheets of 0.3 mg. Cr/dm. in the hydrated chromium oxide and 1.2 mg./dm. metallic chromium were immersed in the solution of the under mentioned composition, and squeezed with the rubber rollers, and dried with the hot air of 80 C. to obtain the product of 1.3 mg./dm. coating amount.

Triazine formalin resin "percent" 50 Temperature C 50 to 60 Example 6 The electrolytic chromate treated steel sheets of 0.4 mg. Cr/dm. in the hydrate chromium oxide and 1.0 mg./dm. metallic chromium were immersed in the solution of the under mentioned composition, and squeezed with the rubber rollers, and dried with the hot air of 80 C. to obtain the products. of 2.1 mg./dm. coating amount.

Titanium modified acrylic resin percent 5 Temperature C 50 to 60 Example 7 The electrolytic chromate treated steel sheets of 0.2 mg. Cr/dm. in the hydrated chromium oxide and 1.0 mg./dm. metallic chromium were immersed in the solution of the under mentioned composition, and squeezed with the rubber rollers, and dried with the hot air of 80 C. to obtain the products of 2.5 mg./dm. coating amount.

Copolymer of itaconic acid and acrylnitrile percent 5 Sodium mol'ybdate ..do..;.. 1 Temperature C 50 to 60 Example 8 The electrolytic chromate treated steel sheets of 0.2 mg. Cr/dm. in the hydrated chromium oxide and 1.0 mg./dm. metallic chromium were immersed in the solution of the under mentioned composition, and squeezed with the rubber rollers, and dried with the hot air of 80 C. to obtain the products of 1.5 mg./drn. coating amount.

Copolymer of itaconic acid and acrylnitrile -percent.... 5 Sodium molybdate do 1 Temperature C 50 to 60 have been effected with the chromate treatment through the electrolysis or immersion.

According to the invention as stated above, the chromum amount in the hydrated chromium oxide is appropriately controlled, to which such a water soluble resin organic film forming agent which does not reduce one or more of such hexavalent chromium as triazine-formalin resin, titanium modified acrylic resin or carboxylic acid modified acrylic resin, is formed to provide enough corrosion resistance and obtain the chromate treated metal sheets with the good paint adhesion and the treatments therefore go well with workings, (with no dangers or rather have the great industrial effects results.

We claim:

1. A chromate treated metal sheet having improved corrosion resistability with workability and paint adherence consisting of a chromate treated metal sheet with a layer of 0.5 to 1.5 mg./dm. metallic chromium on said sheet, a layer of hydrated chromium oxide of 0.11 to 0.8 mg. Cr/dm. on said first layer, and, a dried coating on said second layer of an organic film of 0.05 to 2 mg./dm. from a solution of water soluble resin of an organic film forming agent which does not reduce hexavalent chromium in one or more of the compounds taken from the group, triazine-formalin resin, titanium modified acrylic resin, carboxylic acid modified acrylic resin.

2. A chromate treated metal sheet as claimed in claim 1 including one or more of dicromate, molybdate, phosphate and oxyacid of salt of transition metal included in said solution.

3. A chromate treated metal sheet as claimed in claim 1 wherein said metallic chromium layer is of 1.0 to 1.2 mg./dr n.

4. A chromate treated metal sheet as claimed in claim 1 wherein said hydrate chromium oxide layer is of 0.15 to 0.3 mg. Cr/dmF.

5. A chromate treated metal sheet as claimed in claim 1 wherein said organic film is of 0.05 to 0.5 mg./dm.

References Cited UNITED STATES PATENTS 3,526,486 9/1970 Smith 204-41 X 3,567,599 3/ 1971 Carter et a1 204-41 3,642,587 2/ 1972 Allen et al 204-41 X 3,679,554 7/1972 Yamagishi et a1. 204-56 R 3,519,495 7/ 1970 Plaxton 148-62 3,484,347 112/1969 Yamagishi et al. 204-56 R 3,053,691 9/1962 Hartman et al 148-62 RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 

